(1) Field of the Invention
This patent is generally related to the control of normally interdependent light transmission characteristics, such as polarization and phase delay in an optical fiber, and more specifically to the control of one of those characteristics essentially independently of the other.
(2) Description of the Prior Art
It is well known that, in many applications involving the transmission of light through an optical fiber, the control of various light transmission characteristics can be important. For example, polarization controllers provide a means for transforming light of an arbitrary polarization to light of constant polarization. Generally these systems deform plural areas of the optical fiber to produce controlled birefringence. A number of references disclose such systems generally for controlling polarization.
U.S. Pat. No. 3,625,589 (1971) to Snitzer discloses apparatus for controlling the propagation characteristics of coherent light within an optical fiber by applying a mechanical stress to at least a section of the optical fiber. The stress varies in accordance with a signal thereby varying a characteristic of coherent light as coherent light propagates through the section. The section is arranged so that the amount of light energy from the source guided by the section from one end to the other throughout the entire length of the section is substantially constant as the mechanical stress varies.
U.S. Pat. No. 4,341,442 (1982) to Johnson discloses a fiber optical transmission filter with a double-refraction element for controlling phase delay. A manufacturing process is disclosed to provide a desired phase delay by introducing a filter with at least one double-refraction element comprising a single-mode optical filter mounted between polarizers. The double-refraction of the optical fiber is sufficiently weak so that the wavelength, within which light beams propagating with orthogonal polarization states in the fiber are mutually delayed by 2.pi., is at least 1 cm. In one embodiment the optical fiber comprises alternating sections which produce linear double-refraction with sections which produce elliptical double-refraction.
U.S. Pat. No. 4,729,622 (1988) to Pavlath discloses a fiber optic polarizer that converts a light wave of an arbitrary polarization propagating in a single mode optical fiber into a wave having a single linear polarization state. The polarization controller couples light of an undesired polarization out of the fiber. A photo detector produces an error signal responsive to light coupled from the fiber. Control circuitry processes the error signal to produce control signals which are input to the polarizer to null the error signal. With the error signal maintained at a minimum value, the light input to the polarizer and, hence, the light output therefrom is a wave having only the desired linear polarization.
U.S. Pat. No. 4,753,507 (1988) to DePaula et al. discloses a piezoelectric load housing and method that includes a fiber squeezer with a frame that applies a preload to an optical fiber to permit variation of birefringence. An appropriate voltage source is connected to a piezoelectric transducer to control the force on the fiber thereby to control the refractive indices of the fiber by means of the photo-elastic effect. The relative positions of legs in the fiber squeezer are adjustable during assembly of the frame to permit application of a preload to the fiber and transducer fiber squeezer.
U.S. Pat. Nos. 4,768,851 (1988) and 4,792,207 (1988) and 4,801,189 (1989) to Shaw et al. disclose various fiber optic coupler embodiments. In each embodiment the coupler comprises a single continuous strand of optical fiber and a device for applying stress to the optical fiber at spaced intervals along the fiber. The stress deforms the fiber and abruptly changes the fiber geometry at the beginning and end of each stressed region. The change in fiber geometry causes coupling of light from the fundamental mode to the second order mode. The coupler, under certain conditions, exhibits polarization dependence, and thus, it may be utilized as a fiber optic polarizer. In addition, the device couples coherently, and may be used as an interferometric system. In the Shaw-207 patent, the waveguides are characterized by two modes of propagation in one fiber. Plural distributed coupling ridges or electrodes mounted adjacent piezoelectric materials are independently driven to apply sinusoidally varying sources to the fiber. In this embodiment the input signal is acoustic energy. In the Shaw-189 patent, a highly birefringent fiber optic waveguide is positioned on a flat polished surface with either of the principal axes of birefringence oriented at an angle, preferably 45.degree., to the vertical. A ridged block is then pressed down on the fiber. The ridges have longitudinal axes transverse to the longitudinal axes of the fiber. The width of the ridge faces is one-half beat length and the spacing between the ridges is also one-half beat length. The stressed regions caused by the ridges produce coupling of light traveling in one polarization mode into the other polarization mode by abrupt shifting of the axes of birefringence at the boundaries of the stressed regions.
U.S. Pat. No. 4,781,425 (1988) to Risk et al. discloses an acoustic-optic frequency shifter having a long interaction region for optical analysis. A variable frequency signal generator drives an acoustic transducer to launch an acoustic wave in contact with the optical fiber. The acoustic frequency is varied over a known range to generate acoustic waves having known wavelengths. An optical signal having an unknown optical wavelength is introduced into one end of the optical fiber in a first polarization mode. The effect of the acoustic wave on the optical signal is to cause coupling of the optical signal with the first polarization mode to a second orthogonal polarization mode. The amount of coupling is dependent on the phase-matching between an acoustic wavelength and an optical beat length. The coupling between the polarization modes is maximum when the acoustic wavelength is equal to the optical beat length. The intensity of the optical signal coupled to the second polarization mode can be measured to determine the optical wavelength corresponding to the acoustic wavelength when the maximum intensity occurs.
U.S. Pat. No. 4,793,676 (1988) to Risk discloses a fiber optic amplitude modulator that couples light between two orthogonal polarization modes of a birefringent fiber. Dynamic coupling is caused by applying synchronized acoustic surface waves to the birefringent fiber in a direction normal to the fiber axis. A static biasing force is applied across the fiber to statically couple 50% of the light input to one polarization mode into another polarization mode. The additional force caused by the acoustic waves causes the fraction of coupled power to vary about the coupling caused by the static force.
U.S. Pat. No. 4,988,169 (1991) to Walker discloses an optical signal control method and apparatus. Four birefringent elements in series are arranged to rotate the state of polarization alternately about orthogonal axes on a Poincare sphere. A controller enables time varying initial and final polarization states to be tracked. It also ensures that the birefringence limits of the elements are never reached by carrying out an adjustment procedure. A reduction by a full revolution for one element can be achieved by varying the transformations of the other elements.
U.S. Pat. No. 5,004,312 (1991) to Shimizu discloses a method for controlling the polarization of light by generating first through fifth birefringences in series along a light propagating medium. The first to fifth birefringences have main axes of 0.degree., 45.degree., 0.degree., 45.degree. and 0.degree. relative to an arbitrary direction orthogonal to a light propagating direction of the medium. The magnitude of the birefringences are changed to change the first to fifth phase differences. In an ordinary polarization control, the second to fourth phase differences are changed. However, one or both of the first and fifth phase differences are changed in a resetting operation for one of the second to fourth phase differences. Consequently, the phase differences are reset without the dependency on polarizations of input light supplied to a polarization controller and output light supplied from the polarization controller.
Collectively the foregoing patents disclose various methods of applying stresses to optical fibers to influence various characteristics. Typically the characteristics are polarization and/or delay. However, these patents also depict the characteristics of polarization and delay as being normally interdependent. That is, varying polarization produces significant delay variations, and, conversely, varying delay changes polarization significantly. The application of a force in this manner produces a non-zero birefringence and a non-zero average phase change. If such a polarization controller is used to control polarization in a fiber optic phase sensor system, the non-zero average phase change introduces unwanted phase noise. Conversely if a phase controller using such forces is applied to a polarization sensitive fiber optic sensor system, the non-zero birefringence change introduces an unwanted polarization noise. None of the references seems to disclose the control of one such normally interdependent light transmission characteristic essentially independently of the other. More specifically, none of the references discloses any procedure for varying either polarization or phase delay without incurring a significant change in the other.